The work described herein consists of an examination, by hybridization analysis, of the kinetics of accumulation and disappearance of vitellogenin mRNA in rooster liver, following a primary injection of estradiol. These studies have been carried out with the sense strand of a cloned vitellogenin cDNA fragment. The results obtained are in general agreement with those obtained previously with cDNA produced by reverse transcription of vitellogenin mRNA. The homogeneity of the cloned hybridization probe, however, has made it possible to demonstrate that prior to treatment with estrogen, the level of vitellogenin mRNA in rooster live is less than one molecule per cell. A 6000 nucleotide long fragment of the native vitellogenin gene has been purified 100-200-fold from total chicken DNA, by a combination of chromatography on RPC-5 and preparative agarose gel electrophoresis. The DNA fraction containing this fragment was ligated to the arms of the EK2 bacteriophage vector lambda WESb lambda gt. The chimeric bacteriophage DNA was packaged in vitro and used to transfect the bacterial host, E. coli DP-50 supF. Clones containing DNA sequences derived from the vitellogenin gene were identified by plaque hybridization to 32P labeled vitellogenin mRNA.